Conventionally, a shift control device in a working vehicle such as a tractor for agricultural work is constructed as disclosed in Japanese Published Unexamined Patent Application No. H05-52254 so that the rotation power of the engine is transmitted to the front and rear wheels through a main speed-changing device and an auxiliary speed-changing device, the main speed-changing device can be automatically or manually switched by driving an actuator, and the auxiliary speed-changing device can be mechanically switched by a shift operation of a shift lever.